Environmental Engineering Reference
In-Depth Information
•
—Centrifugal pump efficiency is
directly related to the head capacity of the pump. The highest per-
formance efficiency is available for only a very small section of the
head-capacity range. When the pump is operated outside of this
optimum range, the efficiency may be greatly reduced.
range of efficiency is narrow
•
Pump may run backwards
—If a centrifugal pump is stopped with-
out closing the discharge line, it may run backwards, because the
pump does not have any built-in mechanism to prevent flow from
moving through the pump in the opposite direction (i.e., from dis-
charge side to suction). If the discharge valve is not closed or the
system does not contain the proper check valves, the flow that was
pumped from the supply tank to the discharge point will imme-
diately flow back to the supply tank when the pump shuts off.
This results in increased power consumption due to the frequent
start-up of the pump to transfer the same liquid from supply to
discharge.
Note:
It is sometimes difficult to tell whether a centrifugal pump is run-
ning forward or backwards because it appears and sounds like it is oper-
ating normally when operating in reverse.
•
Pump speed is difficult to adjust
—Centrifugal pump speed cannot
usually be adjusted without the use of additional equipment, such
as speed-reducing or speed-increasing gears or special drive units.
Because the speed of the pump is directly related to the discharge
capacity of the pump, the primary method available to adjust the
output of the pump other than a valve on the discharge line is to
adjust the speed of the impeller. Unlike some other types of pumps,
the delivery of the centrifugal pump cannot be adjusted by chang-
ing some operating parameter of the pump.
3.3.6 Centrifugal Pump applications
The centrifugal pump is probably the most widely used pump avail-
able at this time because of its simplicity of design and wide-ranging
diversity (it can be adjusted to suit a multitude of applications). Proper
selection of the pump components (e.g., impeller, casing) and construc-
tion materials can produce a centrifugal pump capable of transporting
not only water but also other materials ranging from material or chemi-
cal slurries to air (centrifugal blowers). To attempt to list all of the vari-
ous applications for the centrifugal pump would exceed the limitations
of this handbook; therefore, our discussion of pump applications is lim-
ited to those that frequently occur in water/wastewater operations.
•
large-volume pumping
—In water/wastewater operations, the pri-
mary use of centrifugal pumps is large-volume pumping. Generally,
in large-volume pumping, low-speed, moderate-head, vertically
shafted pumps are used. Centrifugal pumps are well suited for
water/wastewater system operations because they can be used in
conditions where high volumes are required and a change in flow
is not a problem. As the discharge pressure on a centrifugal pump
